Small jet engines are widely used in aviation. Among other applications, small jet engines serve as main thrust units for missiles and unmanned air vehicles (UAV's), as auxiliary power units (APUs) for large airborne systems, for example to assist in starting operation of the main engines and in supplying energy for air conditioning systems.
Many small jet engines suffer from poor performance resulting from a low pressure ratio due to limited rotating speed and material strength. The specific fuel consumption (SFC), which is the ratio of fuel that is consumed per time to the thrust that is generated, is conventionally reduced by bypassing a portion of the air introduced to the engine core. The engine core generally comprises a compressor, a combustion chamber in which the compressed air is heated by means of the fuel combustion, a turbine from which the thermal energy of the combustion gases is extracted to drive the compressor, and the exhaust nozzle to provide thrust.
An engine generating a bypass stream of air is called a turbofan engine or a bypass jet engine, and is generally configured with two separate coaxial shafts, in order to match the work output of two compressor units throughout the entire range of their rotational speeds. On each of these shafts are mounted compressor wheels and turbine wheels. A low pressure compressor located immediately after the air intake, generally called a fan, is driven by a low pressure turbine and compresses all of the air introduced to the engine by a compression ratio of 1.5-2.5. One portion of the compressed air is delivered to a high pressure compressor driven by a high pressure turbine and is then directed to the combustion chamber. The combustion gases exiting the combustion chamber cause both turbine wheels to rotate and are then discharged from an exhaust nozzle. The remaining portion of the air compressed by the fan is bypassed through a bypass duct and is mixed in the exhaust section with the core gas flow, although both flows may also be separated to a certain extent. By providing a stream of bypassed air in addition to the core gas flow, a turbofan engine is able to generate an increased level of thrust, i.e. the product of velocity and mass flow rate, often greater than a turbojet engine, at a decreased velocity, resulting in reduced specific fuel consumption. The velocity of the bypassed air may be designed to be close to the velocity of the core gas. Due to the addition of the bypass duct, the outer diameter of a turbofan engine is larger than a turbojet providing the same thrust.
Prior art turbofan engine configurations, particularly for small, low thrust engines of up to a few hundred N, have serious design limitations in terms of an increased outer diameter in order to accommodate the bypass duct. Such small engines can produce a suitably high level of thrust only at high rotational speeds of up to hundreds of thousand RPM. At such speeds, only small bearings can retain their structural integrity, and therefore the inner shaft is required to also have a small diameter. As a result, the small-diameter shaft has low stiffness and high amplitude of oscillations at critical speeds, leading to a difficulty in preventing contact between the two rotating coaxial shafts.
During normal operation of a prior art turbofan engine, the slightly compressed air exiting the fan is directed to stator vanes located between the fan and the compressor. These expensive to manufacture stator vanes are adapted to guide the airflow to the compressor and to perform aerodynamic matching, i.e. causing the velocity vector, which is dependent upon the velocity and angle, of the airflow exiting the fan to change to a suitable velocity vector at the inlet of the compressor which prevents flow separation from the compressor blades. During those situations when the fan is subjected to a relatively high load, e.g. in order to generate a relatively high pressure ratio, two rows of guide vanes are usually required, further increasing costs.
When a one-spool engine configuration is employed, the engine optimally operates at a single designed working point; however, when working at off-design points, the relation between airflow velocity and inlet angle is not optimal. In order to operate optimally with respect to off-design points, it is necessary to install variable-angle guide vanes, or to perform air bleeding between the fan and compressor.
Another disadvantage associated with the use of guide vanes is that a boundary layer is being developed at the leading edge of the vane upon introduction of the airflow to the guide vane. Since the majority of friction losses are related to the developing thin boundary layer, each row of guide vanes causes additional fluid energy losses.
It would therefore be desirable to provide a new turbofan engine configuration that could overcome these drawbacks.
Various lightweight and low cost turbofan engine configurations have been proposed in the prior art.
U.S. Pat. No. 3,937,013 discloses a bypass type jet propulsion engine that includes a core engine with a centrifugal compressor and a centripetal turbine connected coaxially back-to-back and combustion apparatus disposed radially outwardly of the turbine. The fan part of the engine is provided by fan blades extending outwardly from the rotor blades of the core engine compressor which discharge through outlet guide vanes into a fan duct lying radially outwardly of the combustion apparatus. The fan outlet guide vanes are variable to substantially close the fan duct for starting the engine. Part of the air flows from the compressor to the combustion apparatus of the core engine through hollow turbine nozzle vanes. In order to accommodate the large radius centrifugal compressor, the bypass duct is configured with sharp turns of close to 90 degrees, leading to large fluid energy losses.
U.S. Pat. No. 5,105,616 discloses a gas turbine engine comprising a centrifugal compressor with radially inner, high pressure blades and radially outer, low pressure blades in order to provide bypass air and to maximize combustion efficiency. Variable guide vanes selectively occlude the entrance ends of the low pressure blades. In addition to the outer contour with sharp turns that leads to high fluid energy losses and a reduction in thrust generation, the configuration of this engine also generates two aerodynamically separated flows, one for each set of compressor blades. The two flows are at different velocities and pressures, reducing the aerodynamic performance of the engine.
In the turbofan engine of U.S. Pat. No. 7,055,306, a combined stage rotor is mounted for common rotation with a shaft, a centrifugal compressor stage, and a turbine stage as a single unitary wheel. Each of the plurality of fan blades of the combined stage rotor for communicating incoming airflow through the fan bypass duct are contiguous with a respective compressor blade for communicating airflow through the core duct. The combined stage rotor is located at a junction between the fan bypass duct and the core duct. Although the combined stage rotor increases fuel efficiency and thrust to allow the turbofan engine to be used for single usage applications, the fan and compressor stages are nevertheless separated, leading to aerodynamically separated flows and an increased engine diameter as a result of the use of a centrifugal compressor that radially discharges the compressed air.
It is an object of the present invention to provide an impeller to be used in a turbofan engine or in an APU having an outer diameter which is significantly smaller than that of prior art engines for a given thrust.
It is an additional object of the present invention to provide an impeller with improved aerodynamic matching relative to prior art fans and compressors.
It is an additional object of the present invention to provide an impeller to be used in a turbofan engine or in an APU which does not require any guide vanes between the fan and compressor sections.
Other objects and advantages of the invention will become apparent as the description proceeds.